This compound, known in the profession under the name F134a, is especially intended to replace dichlorodifluoromethane (F12) currently used as a refrigerating fluid but suspected of contributing to the depletion of the stratospheric ozone layer. In order to do this, F134a must satisfy quality standards with respect to the presence of a priori toxic impurities, such as chlorofluorinated olefins.
Now, one of the industrial syntheses of F134a consists of a gas phase catalytic fluorination of trichloroethylene or of 1-chloro-2,2,2-trifluoroethane (F133a) which always produces, as a by-product, variable quantities of 1-chloro-2,2-difluoroethylene (F1122) which, given its boiling point (-17.7.degree. C.), proves to be very difficult to completely remove from F134a (B.p.=-26.5.degree. C.) by simple distillation, especially under pressure.
The F134a obtained according to this process or according to other processes generally further contains other olefinic compounds such as fluorinated propenes or butenes. The C.sub.4 olefins such as CF.sub.3 CF.dbd.CF-CF.sub.3 (F1318), CF.sub.3 CF.dbd.CHCF.sub.3 (F1327), CF.sub.3 CH.dbd.CClCF.sub.3 (F1326) and CF.sub.3 .dbd.CHCF.sub.3 (F1336) are not particularly troublesome because they can be separated from F134a by distillation. F134a obtained industrially, in particular that obtained by gas phase fluorination of trichloroethylene or of F133a, generally contains more or less significant quantities of polyfluoropropenes such as CF.sub.3 CH.dbd.CH.sub.2 (1243), CF.sub.3 CF.dbd.CH.sub.2 (1234), CF-CF.dbd.CHF (1225) or their isomers which are very difficult to separate from F134a by distillation because their boiling points are very close to that of 134a.
Many processes for the purification of F134a, and in particular for removing F1122 in F134a, have already been proposed. Mention may thus be made of:
catalytic hydrogenation of F1122 and/or of other fluorinated olefins (WO 9008750, JP 02273634 or JP 04095037); PA1 adsorption of the impurities on active charcoal (EP 389334) or on molecular sieves (U.S. Pat. Nos. 4,906,796, JP 03072437, EP 503,796, EP 511,612 or EP 526,002); PA1 oxidation of F1122 by aqueous potassium permanganate (U.S. Pat. No. 4,129,603).
None of these processes is entirely satisfactory from the industrial viewpoint. Thus, the treatment with aqueous permanganate requires drying the F134a after the purification, which greatly increases the cost of this treatment. Physical adsorption on charcoal or molecular sieve can only be industrially envisaged for a finishing treatment because, taking into account the adsorption capacities of the proposed materials, it appears entirely uneconomical to treat products containing more than a few tens of ppm of adsorbable impurities. Moreover, according to the documents cited above, these adsorption techniques only make it possible to remove F1122 and not C.sub.3 or C.sub.4 olefins. Catalytic hydrogenation requires special plants (compatible with hydrogen) which can only be envisaged industrially if the F134a has itself already been obtained according to a hydrogenolysis process.
Other proposed processes, such as fluorination of the olefins with elemental fluorine (EP 548,744), are also all industrially unadvantageous.
A more advantageous technique is that described in U.S. Pat. No. 4,158,675, which relates to a process for the removal of F1122 consisting in reacting the gases resulting from the main reaction: EQU F133a+HF.revreaction.F134a+HCl
without separation of HCl, HF or unconverted F133a, in a second reactor maintained at a lower temperature than that of the main reaction. From a gas mixture whose F1122 content, relative to the organic compounds, is 5,300 vpm (volume per million), the in-line treatment at 160.degree. C. leads to an F1122 value of 7 vpm.
In this process, the impurity removed (F1122) is refluorinated to F133a, that is to say to a recyclable product. However, the major disadvantage of this process lies in the necessity of having to treat a significant gas flow rate and thus resulting in a high reaction volume, which leads to a prohibitive investment and a prohibitive maintenance cost. Moreover, the process also only relates to the removal of F1122.
In order to avoid these disadvantages, it has been proposed to treat a gaseous mixture of crude F134a and HF in the gas phase, in the absence of hydrochloric acid, in the presence of a fluorination catalyst (JP 04321632, EP 548,742 and Application FR 9,209,700). During this treatment, hydrofluoric acid is added to F1122 and certain other (chloro)fluorinated olefins, such as CF.sub.2 .dbd.CFH (F1123) or CF.sub.3 CH.dbd.CHCF.sub.3 (F1336), and converts them to saturated compounds which are easier to separate and/or recycle by distillation. This process is particularly elegant but, unfortunately, it has been observed that, while F1122 is particularly easy to remove according to this method, other fluorinated olefins such as, for example, the fluoropropenes F1243, 1234 and 1225 have very little reactivity under these conditions and cannot be entirely removed according to this process.